return a callback function which invokes the app1SegmentHandlerBlock if defined.
This will be called to handle the exif segment, containing geolocation tags.
Return nil if there is no handler block defined

read a JPG image from a stream.
For now, we can only read from a stdio-FILE with libjpeg
(need to write a mem-reader to read from a Smalltalk stream).
Therefore, any internal stream data is copied to a temporary file first,
and libjpg asked to decompress from there.
This should be fixed, if jpeg reading is a bottleneck in your app.